like plasmon resonances of single gold nanowires in the mid infrared

Antenna-like plasmon resonancesof single gold nanowires in themid-infraredSERS roundtable 2006Frank NeubrechKirchhoff-Institut für PhysikUniversität Heidelberg

Outline IntroductionExperimental setupNano-antennas Experimental results Outlook Summary

Outline IntroductionExperimental setupNano-antennas Experimental results Outlook Summary

Introduction:experimental setupSpectroscopic IR-microscopy at the synchrotron lightsource ANKA [ * ]IR-microscope (Bruker IRscope II):optical path of the synchrotron-beamcameraapertureMCT“Schwarz-schild”-objective(N.A. 0.52)[* ] ANKA : Angströmsource Karlsruhe,Forschungszentrum KarlsruheFTIR-spectrometerlight source (visible)samplepolarizer

Introduction:measurement principlesynchrotronlight sourceIR beam8µm spotsizeRelative transmittance measurementsMCT detectornanowiresubstratetransmittance T0.0200.0150.0100.0050.000wavelength [µm]10.00 5.00 3.33 2.50 2.00 1.67 1.43sample1000 2000 3000 4000 5000 6000 7000wavenumber [cm -1 ]spectrometer1.02wavelength [µm]10.00 5.00 3.33 2.50 2.00 1.67 1.430.020wavelength [µm]10.00 5.00 3.33 2.50 2.00 1.67 1.43relative transmittance ( T / T 0)1.000.980.960.941000 2000 3000 4000 5000 6000 7000wavenumber [cm -1 ]diameter D = 100nm,length L = 1.4µm,substrate: ZnStransmittance T 00.0150.0100.0050.000reference1000 2000 3000 4000 5000 6000 7000wavenumber [cm -1 ]

Introduction: nano-antennasλEasiest model: macroscopic ideal antenna L = j ⋅ e.g. ω λ / 2=From measurements [ * ] 2⋅nand light scattering simulations of goldnanowires in the visible spectral range deviations are observedπcj ⋅L ⋅ nL ≈λ3Boundary element method (BEM) simulations [#]Deviations due to retardation, skin effect, influence of substrate andsurrounding material⎛ D ⎞⎛ D ⎞ πc⎛ D ⎞ωres⎜L, neff, ⎟ = ωλ / 2( L,neff) ⋅ R⎜⎟ = ⋅ R⎜⎟⎝ L ⎠⎝ L ⎠ L ⋅ n ⎝ L ⎠eff[* ] G. Schider, J. R. Krenn et al., Phys. Rev. B 68, 155427 (2003); [#] J. Aizpurua, G.W Bryant et al., Phys. Rev. B 71, 235420 (2005)

Outline IntroductionExperimental setupNano-antennas Experimental results Outlook Summary

Experimental resultsMeasured relative transmittance spectra of single nanowireswith different lengths and diameters on different substrates [ * ]Picture of a nanowiretaken with the IRmicroscopein thevisible mode10µm x 10µm[ * ] F. Neubrech, T. Kolb, R. Lovrincic, G. Fahsold, J. Aizpurua, S. Karim, T. W. Cornelius,M. E. Toimil-Molares, R. Neumann and A.Pucci, Applied Physics Letters (submitted)

Experimental results:Resonance wavelength (Γ)Influence of the substrateEffective medium:resonance wavelength [µm]1412108642D = 200nm (exp.)D = 210nm (exp.)dipole (n = 1)dipole (n = n eff= 1.29)substrate:KBr1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0wire length [µm][ * ] Kreibig, U. und M. Vollmer: Optical properties of metal clusters. Springer, 1995 Rough approximation [ * ] :substrateLeff( ε ε )1ε ≈ + 2subsurModified λ/2- relation=Γ2⋅neffε < ε < εair,effneffnanowire≈subεeff

Experimental results:Resonance wavelength (Γ) Resonance wavelength (Γ) vs. wire length (L) [ * ]resonance wavelength [µm]14 BEM calculations (D = 200nm, n = 1)BEM calculations (D = 200nm, n eff= 1.29)dipole (n = 1)12dipole (n = n eff= 1.29)D = 200nm (exp.)10D = 210nm (exp.)86421.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0wire length [µm][ * ] F. Neubrech, T. Kolb, R. Lovrincic, G. Fahsold, J. Aizpurua, S. Karim, T. W. Cornelius,M. E. Toimil-Molares, R. Neumann and A.Pucci, Applied Physics Letters (submitted)Light scatteringsimulations (performedby J. Aizpurua) of goldnanowires considering wire aspect ratio material propertiesn eff=1 (air)Scattering simulationsin considering n eff are ingood accordance withour measured data

Experimental results:surrounding medium Influence of the surrounding medium on ω resbefore evaporating paraffinafter evaporating paraffin1sample referencesample referencenanowire2paraffinnanowiresubstrate (ZnS)substrate (ZnS)surrounding medium: airsurrounding medium: paraffin

Experimental results:surrounding mediumShift of ω res due to the polarizability of paraffinrelative transmittance1.000.960.9210 7 5 4 3 3 3diameter D = 100nm, length L = 1.4µm, substrate ZnSparω resω res2850 cm -12919 cm -1before evaporationafter evaporationEstimation of the ratiopar ⎛ D ⎞parπc/( Lneff) ⋅ R⎜⎟ωnresL=⎝ ⎠=ωresair ⎛ D ⎞ nπc/( Lneff) ⋅ R⎜⎟⎝ L ⎠usingsystem 1systemεeff≈ ( εsub+ εsur), neff≈2aireffpareffεsystemeff1000 1500 2000 2500 3000 3500 4000wavenumber [cm -1 ]Exp. Value: 0.92 +/- 0.03ω⇒ωparresres=nnaireffpareff= 0.92ε ZnS4 .84, ε = 2.02=par

Experimental results:extinction cross sectionFrom relative transmittance to the extinction crosssection related to the geometric cross section1.0030relative transmittance0.950.900.85diameter D = 210 nmlength L = 2.37 µmparallel polarizationσ ext(ω) / σ geo(ω)25201510diameter 210 nm, length 2.37 µmmeasurement (p-polarized)0.801000 1500 2000 2500 3000 3500 4000 4500wavenumber [cm -1 ]5σσextgeo= A ⋅(1−T( ω))⋅01LD⋅(1 + n2sub)0500 1000 1500 2000 2500 3000 3500 4000 4500wavenumber [cm -1 ]spot sizeextinctiongeometriccross sectionsubstrate effect

Experimental results:extinction cross sectionσ ext(ω) / σ geo25201510Enhancement of the extinction cross sectionwavelength [µm]20.0 10.0 6.67 5.00 4.00 3.33 2.86 2.50 2.2230diameter D = 210 nm,length L = 2.37 µm,p-polarized,substrate: KBrRatio σ ext / σ geo of asingle nanowire at theresonance maximum:/ ≈ 25σ extσ geo50500 1000 1500 2000 2500 3000 3500 4000 4500wavenumber [cm -1 ] Indication of localfieldenhancement inthe vicinity of the wire

Experimental results:extinction cross sectionLight scattering simulationswavelength [µm]σ ext(ω) / σ geo20,0 10,0 6,67 5,00 4,00 3,33 2,86 2,50 2,223025201510Ekdiameter 210 nm, length 2.37 µmmeasurement (p-polarized)BEM calculation (n eff=1.29)Light scatteringsimulations (boundaryelement method (BEM))performed by J.Aizpurua n eff = 1.2950500 1000 1500 2000 2500 3000 3500 4000 4500wavenumber [cm -1 ] Good accordance withlight scatteringsimulations performedby J. Aizpurua

Outline IntroductionExperimental setupNano-antennas Experimental results Outlook Summary

Outlook: SEIRASurface enhanced infrared absorptionOccurs for adsorbates on rough films, island films,nanowires,…Enhancement due to electrical ‘field enhancement’(in the near field of metal nanostructures),…SEIRA-studies of arrays of gold nanowiresSEIRA-studies of slits in gold nanowiresEnhancement up to a factor of 200 000 isexpected [ * ][ * ] J. Aizpurua, G.W Bryant et al., Phys. Rev. B 71, 235420 (2005)

Outlookfirst SEIRA measurementsAdsorbate: octadecanthiol (ODT) with characteristic CHstretching vibration bands (2848 cm -1 and 2915 cm -1 [ * ] ) Adsorbent: single gold nanowire deposited on CaF 2ODT(1 monolayer)nanowiresubstrate (CaF 2)samplereference[* ] D. Enders and A.Pucci, Applied Physics Letters, 88, 184104 (2006)

OutlookFirst SEIRA measurementsRelative transmittance spectra6.67 5.00 4.00 3.33 2.86 2.50 2.22 2.00relative transmittance1.000.980.960.940.922850 cm -12919 cm -10.4 %diameter D = 100nmlength L = 1.7µmsubstrate: CaF 2no polarizer used1500 2000 2500 3000 3500 4000 4500 5000--wavenumber [cm -1 ]

OutlookFirst SEIRA measurementsRelative transmittance spectra6.67 5.00 4.00 3.33 2.86 2.50 2.22 2.00relative transmittance1.000.980.960.940.922850 cm -12919 cm -10.4 %diameter D = 100nmlength L = 1.7µmsubstrate: CaF 2no polarizer used0.4 %2850 cm -12919 cm -11500 2000 2500 3000 3500 4000 4500 5000--wavenumber [cm -1 ]2800 2900 3000wavenumber [cm -1 ] Possible enhancement

Outline IntroductionExperimental setupNano-antennas Experimental results Outlook Summary

SummarySpectroscopic IR–microscopy at single goldnanowires: resonancesω res , Γ : influenced by shape, substrate andsurrounding mediumσ ext / σ geo : Enhanced far-field cross section,which indicates an enhanced local fieldin the vicinity of the nanowireSEIRA We performed primarily SEIRA studies of ODT (1monolayer) on single gold nanowires In order to maximize the SEIRA, several SEIRAmeasurements with gold nanowires are planned

AcknowledgementKirchhoff-Institute for physicsOur Group, Kirchhoff-Institut für Physik, Universität Heidelberg, GermanyA. Pucci (group leader)G. Fahsold (former member)T. Kolb (former member)O. SkibbeR. LovrincicM. KlevenzF. MengM. BinderMaterials Research Department, GSI, Darmstadt, GermanyR. Neumann (group leader)E. Toimil-Molares (former member)S. KarimT. CorneliusDonostia International Physics Center, San Sebastian, SpainJ. AizpuruaEmployees at the ANKA IR-beamline, Forschungszentrum Karlsruhe,GermanyY.L. MathisM. Suepfle

END

Introduction: preparationThe gold nano-wires are prepared by electrochemicaldeposition in polymeric etched ion track membranes atthe Material Research Department at the GSI [ * ]Preparation process:irradiation etching metallic layer electro-deposition dissolution[* ] GSI : Gesellschaft fürSchwerionenforschung, DarmstadtL ~ 1.7µmSEM image of a goldnano-wire (diameterD=100nm, lengthL=1.7µm substrate:silicon)

SEM imagesAu- nanowires (100nm) at SiHRTEM and SAED images50 nm 20 nmLiu, J., Karim. S et al. Synthesis of gold nanowires innanoporous ion track membranesL ~ 1.7µmInvestigated nanowires• 1-5µm long• diameter 80-200 nm• single- and polycristalline

EM- Scattering at small particlesconsider a small spherical particle with D

Quasistatic LimitωωPLDaspectratio q=L/D13nanoparticle(Mie resonance)1 L/D ∝Bohren, C. und D. Huffmann: Absorption an scattering oflight by small particles. John Wiley and Sons, 1983.No longer quasistatic, we have to regard retardation

Introduction: nano-antennasλπEasiest model: macroscopic ideal antenna L = j ⋅ e.g. ω λ / 2= j ⋅2⋅nL ⋅From measurements and light scattering simulations of nanowires inthe visible spectral range deviations are observedcnL = λ2 ⋅nLkEMeasurements at gold nano-rods [ * ]Boundary element method (BEM) simulations [#]Deviations due to retardation, skin effect, influence of substrate andsurrounding material⎛ D ⎞⎛ D ⎞ πc⎛ D ⎞ωres⎜L, neff, ⎟ = ωλ / 2( L,neff) ⋅ R⎜⎟ = ⋅ R⎜⎟⎝ L ⎠⎝ L ⎠ L ⋅ n ⎝ L ⎠eff[* ] G. Schider, J. R. Krenn et al., Phys. Rev. B 68, 155427 (2003); [#] J. Aizpurua, G.W Bryant et al., Phys. Rev. B 71, 235420 (2005)

Experimental results:extinction cross sectionσ ext(ω) / σ geoEnhancement of the extinction cross section20.0 10.0 6.67 5.00 4.00 3.33 2.86 2.50 2.2250403020100Ekdiameter 210 nm, length 2.37 µmmeasurement (p-polarized)MWS calculation (p. c. cylinder)MWS calculation (lossy metal)BEM calculation (n = 1)BEM calculation (n eff=1.29)BEM calculation (L = 2.15 µm,neff= 1.29)500 1000 1500 2000 2500 3000 3500 4000 4500wavenumber [cm -1 ]Finite difference timedomain (FDTD)simulationslight scatteringsimulations (boundaryelement method (BEM))performed by J. Aizpurua n eff = 1 (vacuum) n eff = 1.29 different lengths Good accordance with exact light scattering simulations performed by J. Aizpurua

Basics: Experimental determination of opticalproperties - thin filmsrelative transmittance at normal incidence:T film/substrateT substrateε film− ε ∞=≈1– 2⋅d⋅ω⋅Imε IIfilm(ω)c⋅(1+n substrate )iσ( ω)ωε 0For films much thinner than the wavelength in the material d

Resonance width (ω Γ )width ω Γ[cm -1 ]700600500400300200100 nm (Au@ZnS)160 nm (Au@CaF)330 nm (Au@CaF)100 nm (Cu@KBr)200 nm (Cu@KBr)80 nm (Au@KBr)100 nm (Au@KBr)200 nm (Au@KBr)210 nm (Au@KBr)200 nm Sim (pc. cyl.)100 nm Sim (pc. cyl.)100600 800 1000 1200 1400 1600 1800 2000 2200 2400resonance frequency [cm -1 ]• in accordance with RCS- calculations (scattering at a perfect conducting wire)• no difference between Au and Cu wires materialproperties are not dominant

Polarisationeffectsexperimental setup:polarizernanowireMCTideal case: (not this wire)E Per1.00substratE Par1 µmrel. transmittance0.950.900.85parallel polarizedperpendicular polarizedreal case: (not this wire)5 µm0.801000 1500 2000 2500 3000wavenumber [cm -1 ]SEM images: Au- nanowires at Si done by S. Karim

What is SEIRA?Surface enhanced infrared absorption,enhancement up to a factor of 2000observed, much more is theoreticallypredictedAnalogous to SERS and in certain casescomplementary (because of differentselection rules)

What is SEIRA?Occurs for adsorbates on metal nanostructures(rough nanofilms, island films, grids,...)Enhancement due to“field enhancement” (in the nearfield of metalparticles, at “hot spots” of disordered metal-islandfilms, at plasmon resonances of periodic structures -see photonics,...)“chemical effect” (increase in polarizability byadsorption)“first layer effect” (non-adiabatic interaction betweenvibrations and electron-hole-pair excitations)

SEIRA EffectThe phenomenon surface enhanced IR absorption (SEIRA) concernsmolecules adsorbed on metal surfaces IR absorption,2µ 2 2∂A ~ E ∂ Qcos•Enhancement mechanism:-Electromagnetic field enhancement: Incident photon interaction withmetal surface enhances local electric field E localat surfaceθ;Eµ =Q==ElectricDipoleNormalfieldmomentCoordinateJ.P.Kottmann, O.J.F. Martin,OPTICS EXPRESS 8 (2001) 655

SEIRA:enhancement factor0.4 %2850 cm -12919 cm -12800 2900 3000wavenumber [cm -1 ] Our case: ~500

Resonance enhancend IR-spectroscopy– the ideaIR- spectra of a single gold nanowire:nearfield calculation:E(1666cm -1 )/E 0(1666cm -1 )relative transmittance1.000.950.900.85diameter 210 nmlength 2.37 µmL=3.0µm, D=50nm0.801000 1500 2000 2500 3000 3500 4000wavenumber [cm -1 ]absorptionbands of paraffin (measured at anka):x [µm]model:y [µm]yxthickness: unknownthe absorptionbands ofparaffin have to be in therange of the resonancefrequency of the nanowire

Resonance enhanced IR- spectroscopyexperimentalsetupexperimental setup:IR- spectra:NW,parpar,KBrKBrparaffinnanowiresubstrate (KBr)procedure:1. rel. Transmisionmeasurement at position[nw, par] and [par,KBr]2. calcutate the ratio p =I(1470cm -1 ) / I(2930cm -1 )3. comparison between p[nw,par] and p [nw,KBr] we are not able to detect any enhancement

Resonance enhanced IR- spectroscopyel. Field enhancement is spacial limitedL=3µm, D=50nmE(1667cm -1 ) /E 0 (1667cm -1 )ratio of enhanced signal tobackground signal is small(~7%)L=2.7µmD=200nmA spot=54.4µm 2 only a small fraction of the evaporatedparaffin perceive the enhanced field only a small fraction ofthe signal is enhancend we are not able to detect any enhancement with this experimental setup

At the sharp nanowire ends and particularly in a small gap betweensuch ends a huge nearfield-field enhancement is expected.CO adsorption on Cu films at 100KSEIRAIsland-film example:Ep vibG. Fahsold, M. Sinther, A. Priebe, S. Diez, and A. Pucci, Phys.Rev B 70 (2004) 115406E local enhanced